The present invention relates to printing apparatus, and more particularly to a device and method which both expedites and facilitates printing of quality images.
The printing of images on articles (such as sheets of textiles, plastic or the like) is commonly accomplished by screen printing machines. Generally, these machines are provided with a conveyorized mechanism such as a printing blanket. The article is placed flat on the blanket and indexed to a printing station. A printing head is then lowered onto the article and a printing operation is performed. For example, a print squeegee is stroked across the surface of a horizontal screen in registry with the article so as to force printing ink through the screen and onto the article, thereby effecting printing.
To assure quality, it has been found desirable to position the printing head and, more particularly, the screen into precise registration with the article upon each printing operation. Precise registration has been found inhibited, however, by manufacturing imperfections in the screens as well as temperature variations during printing, particularly those causing expansion and contraction of the screen.
To compensate, micro-registration devices have been used to adjust the screens (along their X-Y axes) into precise registration with the article. To effect these adjustments, it was often necessary to halt printing operations. After changing articles or designs, it also became necessary to reposition the screen. Either way, a time-consuming process was needed which involved back and forth, iterative adjustments to both ends of the screen. An objective was to insure that the screen was in precise registration with the article upon each step of the printing operation.
Although devices with reference lines, calibration gauges or centering targets have been used in an effort to simplify this task, i.e., zero or reset the screen to a selected starting position prior to adjustment, they have been found inherently inaccurate for use in many printing applications. In particular, alignment of these devices has been found to vary not only with the position from which they are viewed, but also with temperature fluctuations during printing. They have also been found lacking in reset capability and ready alignment of both ends of the screen, delaying both set-up and production of printed articles.